8.7 Heat Dissipation with Ventilation
317
where P is the barometric pressure (kPa) in the gallery.
(c) Calculation of airflow rate (Q) (m
3 s
−1 ) (Eq. 8.29):
Q =
q eo
ρ(S o − S i )
(8.29)
where
• ρ: Air density (kg m
−3 ),
• q eo : Rate o heat flow to the air along the airway (kW),
• S i : Air sigma heat at the inlet (kJ kg
−1 ), and
• S o : Highest acceptable value of sigma heat in the air leaving the outlet (kJ kg
−1 ).
Exercise 8.7 The exhaust gases from diesel equipment located in a cul-de-sac are
evacuated by means of a forcing ventilation system. The average wet-bulb temperature of the clean gases is 21 °C. The working power of the diesel equipment is
520 kW. The barometric pressure in the cul-de-sac is 111 kPa. Determine the airflow
rate necessary for the gases to return to the main gallery at an average wet-bulb
temperature of 29 °C.
Solution
Following the procedure outlined in the previous section:
1. Calculate the saturated vapour pressure (P swi ) for the air at the wet-bulb
temperature of the inlet (t wi ):
P swi = 0.6106 e (
17.27·21
21+237.3 ) = 2.49 kPa
2. Determine sigma heat (S i ):
S i = 0.622
2.49
111 − 2.49
(2502.5 − 2.386 · 21) + 1.005 · 21 = 56.11
kJ
kg dry air
3–4. Calculate the saturated vapour pressure (P swo ) for gases at the maximum
allowable temperature (t wo ):
P swo = 0.6106 e (
17.27·29
29+237.3 ) = 4.00 kPa
5. Recalculate sigma heat for saturated pressure (S o ):
S o = 0.622
4.00
111 − 4.00
(2502.5 − 2.386 · 29) + 1.005 · 29 = 85.72
kJ
kg dry air
6. Calculate the required airflow rate based on the total heat flow to the air (q eo ):